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The ρ value for this equilibrium is an estimate based on the correlation of gas-phase ∆Hacid values9 of different oxygen containing acids and is done as follows: The Hammett ρ value for phenol dissociation equilibriums in aqueous solution is 2.217. The gas-phase ρ value for this same equilibrium is 1.289. Hence the ratio (ρaqgas) is 1.73. The Hammett ρ value from the correlation of gas-phase ∆Hacid values of benzyl alcohols versus those of phenyl acetic acids is 1.009. Therefore the Hammett ρ value for benzyl alcohols dissociation equilibriums in aqueous solution on benzoic acid dissociation scale would be: (ρ of phenyl acetic acids) x (ρaqgas) = 1.00 x 1.73 = 1.73.

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Article

Attenuation Effect in Twenty One Different Proton Dissociation Equilibriums Brought on One Rope: A Chemical Education Tool for Evaluation of pKa of Proton Dissociation Equilibrium of Any Substituted Benzene (XC6H5)

1Department of Chemistry, Geethanjali College of Engineering and Technology, Cheeryal-501301, Telangana, India

2Department of Chemistry, Osmania University, Hyderabad-500007, India


World Journal of Chemical Education. 2020, Vol. 8 No. 2, 61-66
DOI: 10.12691/wjce-8-2-1
Copyright © 2020 Science and Education Publishing

Cite this paper:
R. Sanjeev, V. Jagannadham. Attenuation Effect in Twenty One Different Proton Dissociation Equilibriums Brought on One Rope: A Chemical Education Tool for Evaluation of pKa of Proton Dissociation Equilibrium of Any Substituted Benzene (XC6H5). World Journal of Chemical Education. 2020; 8(2):61-66. doi: 10.12691/wjce-8-2-1.

Correspondence to: V.  Jagannadham, Department of Chemistry, Osmania University, Hyderabad-500007, India. Email: jagannadham1950@yahoo.com

Abstract

The strong empirical relation, ρ = (2.4)(2-i), between the Hammett ρ for proton dissociation of several acids and the number, “i”, of atoms between the ionizable hydrogen and the ring carbon (Andrew Williams, Free Energy Relationships in Organic and Bioorganic Chemistry, Royal Society of Chemistry, Cambridge, 2003, p. 75) is used to construct a graph for twenty one different proton dissociation equilibriums. The plot of Hammett ρ versus number of atoms i between ionizable hydrogen and the ring carbon atom is observed to be an excellent exponential-decay locus. A good average and intelligent value of Hammett ρ is obtained for the benzene dissociation equilibriums by interpolating the locus of the correlation on to Y-axis. Using this Hammett ρ value and the Hammett equation log [(Ka)X / (Ka)H] = ρσ, the pKa value can be calculated for any substituted benzene knowing the pKa value of benzene to be 43. The points for proton dissociation equilibriums of phenylethyl ammonium ions and benzyl alcohols deviated from the graph hence not included in the correlation. Possible explanations are given for deviation of these two equilibriums.

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